1. Field of the Invention
The present invention relates to a zoom lens with a wide angle of view and a high magnification, suitable for an image pickup apparatus such as a digital still camera, a video camera, or a television camera, etc.
2. Description of Related Art
Conventionally, a zoom lens comprising, in order from the object side, a first lens unit with positive optical power, a second lens unit with negative optical power, having a magnification varying function, and at least one other lens unit has been known. In this zoom lens, the first lens unit is provided with a retro focus type optical arrangement including a first lens component having negative optical power and a second lens component having positive optical power, wherein the first lens component has, in order from the object side, one negative lens element whose concave surface is faced to the object side, and one or more lens elements. Such a zoom lens is disclosed in Japanese Patent Application Laid-Open No. H08(1996)-184758 (corresponding to U.S. Pat. No. 5,831,771).
In such a zoom lens, since the back side principal point of the first lens unit is pushed out toward the image plane side, the effective diameter of the first lens unit can be reduced to be comparatively small. Therefore, this zoom lens is frequently used as a wide-angle and high-magnification zoom lens whose zooming ratio exceeds 10 times.
Herein, for the zoom lens, reduction in size and weight has been strongly demanded as well as achievement of both a wide angle of view and a high magnification.
However, in the conventional structure such as disclosed in Japanese Patent Application Laid-Open No. H08(1996)-184758 (corresponding to U.S. Pat. No. 5,831,771), when it is attempted to realize a smaller size while achieving a wider angle of view, it becomes necessary to increase the negative optical power of the first lens component and the positive optical power of the second lens component.
If the positive optical power of the second lens component increases, positive distortion increases at a zooming position (focal length) of fwxc3x97Z1/4 provided that the focal length at the wide-angle end is fw and the zooming ratio is Z.
Furthermore, in addition to the increase in the negative optical power of the first lens component, in a case where the positive distortion is corrected, on the assumption that the maximum height of the maximum image height off-axis light ray at the wide-angle end in a condition where the object distance is infinity is defined as hw, and the same at a zooming position of fwxc3x97Z1/4 is defined as hz, the radius of curvature of the first surface (surface of the object side) of the negative lens element in the first lens component, satisfying hw less than hz, becomes smaller.
As a result, the light ray incident angle onto the first surface of the negative lens element in the first lens unit increases, and distortion components in chromatic aberration of magnification significantly increase although positive distortion is satisfactorily corrected.
In order to correct this, it is necessary to make Abbe""s number of the negative lens element in the first lens unit larger, however, this results in insufficient correction of axial chromatic aberration at the telephoto end, and it becomes difficult to achieve a wider angle and a higher magnification while maintaining high optical performance, furthermore, a smaller size and a lighter weight.
An object of the invention is to provide a small-sized lightweight zoom lens with a wide angle of view and a high magnification, having high optical performance by setting the lens structure and conditions properly.
In order to achieve the abovementioned object, according to one aspect of the invention, a zoom lens comprises, in order from the object side, a first lens unit having positive optical power, a second lens unit having negative optical power and has a magnification varying function, and at least one other lens unit. The first lens unit includes, in order from the object side, a first negative lens element whose object side surface is concave toward the object side, and a second negative lens element, and at least one positive lens element is closer to the image plane side than the second negative lens element. And the zoom lens satisfies the following condition:
xe2x88x921.28 less than fn/f1
where fn denotes the composite focal length of the first negative lens element and the second negative lens element, and f1 denotes the focal length of the first lens unit.
The characteristics of a zoom lens and an image pickup apparatus of the invention become clear by the following detailed description with reference to the accompanying drawings.